For me, I ALWAYS get a new ticket number and have ALWAYS had the same PNR. The only slight irregularity seems to be with this new waitlisting that's becoming common. My latest upgrade changed to I class a couple of hours after I called them and was waitlisted, but the ticket number remained the same. It was only the next day when presumably somebody manually re-ticketed it that the actual ticket number changed and I received the email of sucessful upgrade. So it seems that he booking changing to I class online does NOT necessarily mean you've been upgraded yet. I've yet to see what happens with a failed waitlist.